The present invention relates to an optical-information recording apparatus, an optical-information reproducing apparatus, an optical-information recording/reproducing apparatus, an optical-information recording method, an optical-information reproducing method, and an optical-information recording/reproducing method.
At present, the Blu-ray Disc™ standard using the blue-violet semiconductor laser has made the about-50-GB-recording-density-equipped optical discs commercially available even in their consumer products. In the coming years, this large-capacity implementation of the optical discs is desired up to the same extent as that of the HDD (:Hard Disk Drive), i.e., 100 GB to 1 TB.
Implementing an ultra-high density like this using the optical discs, however, requires a high-density-implementing technology which is based on some new scheme, and which is different from the conventional high-density-implementing technology. Here, this conventional technology is based on the shorter-wavelength implementation and the objective lens' high-NA implementation.
Various researches about the next-generation storage technologies are underway at present. Of these technologies, attention is focused on the holographic recording technology for recording digital information by taking advantage of holography.
The holographic recording technology is the following technology: Namely, a signal beam is equipped with information on page data which is modulated in a two-dimensional manner by a spatial light modulator. Then, this signal beam is superimposed on a reference beam inside a storage medium. At this time, an interference-fringe pattern is formed as a result of this superimposition. Moreover, the information is recorded into the storage medium in such a manner that a refractive-index modulation is caused to occur inside the storage medium by this interference-fringe pattern.
At the time of reproducing the information, the storage medium is illuminated with the reference beam used at the time of recording the information. As a result, the hologram recorded into the storage medium operates like a diffraction grating, thereby generating a diffracted light. This diffracted light is reproduced as the same light, including the signal beam and phase information recorded.
The reproduced signal beam is detected in a two-dimensional manner at a high speed, using an optical detector such as CMOS or CCD. In this way, the holographic recording technology makes it possible to record two-dimensional information at one stroke into an optical storage medium using a single hologram. Moreover, this technology makes it possible to reproduce this two-dimensional information recorded. Furthermore, this technology makes it possible to overwrite plural pieces of page data at a certain location of the storage medium. These features allow accomplishment of large-capacity and high-speed recording/reproduction of information.
As a holographic recording/reproducing technology as is described above, there exists, e.g., JP-A-2010-61718. In JP-A-2010-61718, the disclosure has been made concerning a technology for solving “the following problem: Namely, in an optical apparatus, while a light beam is traveling along its optical path, the wavefront of the light beam is deformed by optical elements such as lenses. Moreover, between mutually different optical apparatuses, even if deployments of the optical systems are made mutually identical to each other, the ways in which the wavefronts of the light beams are deformed become different from each other. Accordingly, in general, between a holographic recording apparatus and a holographic reproducing apparatus as well, the wavefront profiles of reference beams become different from each other. On account of this, between the signal recording time and the signal reproducing time, the wavefront profiles of the reference beams do not become one and the same wavefront profile. As a result, the SNR (i.e., SN ratio) of the reproduced data becomes lowered.” by employing “the following configuration: Namely, along each of optical paths of the reference beams, there are provided and arranged an optical detector for detecting the wavefront of a reference beam, and a wavefront controller for adjusting the wavefront of the reference beam. In the wavefront controller, the wavefront profile of the reference beam detected by the optical detector is adjusted so that this wavefront profile becomes the wavefront profile of the reference beam used at the signal recording time into the holographic storage medium”: